The present invention relates to a method for recovering diphenylcarbonate in substantially pure form from a solution of diphenylcarbonate and phenol. More particularly, the present invention relates to the formation of a crystalline 1:1 molar adduct of diphenylcarbonate and phenol in a solution of diphenylcarbonate and phenol, the isolation of such 1:1 molar adduct from such phenolic solution, and the recovery of diphenylcarbonate therefrom.
Prior to the present invention, diphenylcarbonate was made by a variety of procedures which resulted in the production of mixtures of diphenylcarbonate and phenol. One procedure involves the phosgenation of phenol in an aqueous environment (slurry or melt), or in a solvent such as methylene chloride. After the removal of sodium chloride, the diphenylcarbonate can be separated from residual phenol by vacuum distillation.
A second route to diphenylcarbonate is by transesterification of dimethylcarbonate with phenol. A more direct procedure for making diphenylcarbonate involves the carbonylation of phenol with carbon monoxide. A transition metal catalyst such as a palladium catalyst is used in the carbonylation route in combination with a quaternary ammonium halide. A further procedure for making diphenylcarbonate involves the reaction between a cycloalkylene carbonate and phenol.
Although various methods have been developed to make diphenylcarbonate, its recovery from a phenol containing reaction mixture in substantially pure form is often difficult to achieve. For example, the carbonylation of phenol can involve the use of a complex palladium catalyst which can include a thermally unstable quaternary ammonium halide which can generate corrosive by-products. Reaction by-products, such as phenyl salicylate and organic cocatalyst, such as benzoquinone, which can be consumed during the course of the reaction can result in additional impurities in the carbonylation reaction mixture. Although separation of unreacted phenol can be effected by distillation from the reaction mixture, the diphenylcarbonate residue can be contaminated with various catalyst ingredients.
An aqueous extraction technique based on the separation of a phenol enriched aqueous phase from a phase enriched with diphenylcarbonate also has been considered. However, it has been found difficult to form both a diphenylcarbonate phase free of water, and an aqueous phenolic phase. Satisfactory procedures for recovering diphenylcarbonate from solutions of diphenylcarbonate and phenol are therefore of significant interest to the plastics industry, since diphenylcarbonate is a valuable intermediate in the syntheses of polycarbonate resin via a transesterification process.